Preheat the oven to 400°F. Grease an ovenproof dish. Heat 1 tbsp oil in a saucepan and fry the beef for 15 mins, until crumbly, adding the carrots, celery and half the onion after 5 mins.,______,To make the bechamel sauce, melt the butter in a separate saucepan, add 2 tbsp flour and cook, stirring, for 2 mins. Gradually add the stock and milk, stirring constantly. Bring to a low boil and simmer for 5 mins. Season with salt and black pepper.,Heat 1 tbsp oil in a separate saucepan and fry the garlic and remaining onion for 2 mins. Add the spinach, cover and cook until wilted. Season with salt and a pinch of nutmeg.,Spread 2 tbsp beef sauce in the bottom of the dish. Spread over the spinach. Spoon the remaining beef sauce into the cannelloni and arrange on top of the spinach. Pour over the bechamel sauce. Arrange the tomato slices on top and sprinkle over the cheese. Bake for 40 mins. Serve garnished with the reserved thyme.